Combustion engines to produce mechanical work through the combustion of a fuel with oxygen have long been known in the art. Particularly well-known modern-day application of such combustion engines are as prime movers for vehicles. Most often in such applications, a liquid or gaseous fuel is injected into an entrained flow of ambient air and is combusted thereby within one or more cylinders of the engine, producing mechanical power transmitted to a rotating shaft and exhaust products to be removed from the engine.
In an effort to improve the power density of the engine (i.e. the engine power produced divided by the swept volume of the engine cylinders), the rate of throughput of air and fuel through the engine can be increased by compressing the air to a higher density. Such boosting of power density can be economically achieved by recapturing otherwise lost energy remaining in the exhaust products after they have been removed from the engine, a process commonly referred to as turbocharging.
Efforts have also been made to reduce the environmental impact of the exhaust products of such engines. The exhaust products often include elevated concentrations of oxides of nitrogen (NOx) formed by reaction between the oxygen and nitrogen within the oxidizing air flow at the elevated temperatures. NOx formation is generally undesirable, as these oxides are known to react with other chemicals to produce ground-level ozone, a health hazard. As one means of reducing such pollutants, a method of operating the combustion engine whereby a portion of the oxygen-depleted exhaust from the engine is recirculated back to the engine cylinders along with the fresh air and fuel. The extra mass of the essentially inert recirculated exhaust gas increases the heat capacity of the gases within the cylinder without impacting the oxygen:fuel ratio, thereby decreasing the peak combustion temperature and reducing the concentration of NOx within the exhaust. In order to further reduce the peak combustion temperatures (to thereby further decrease the concentration of NOx), both the recirculated exhaust gas and the compressed combustion air are commonly cooled to reduce their temperature prior to their entry into the engine. As an additional benefit, the density of the air and recirculated exhaust gas is also increased by reducing their temperatures, leading to further increase in engine power density and improved fuel economy.
A typical system of the type described is depicted in schematic fashion in FIG. 1. Such an engine system 51 includes an engine 52 having an intake manifold 53 and an exhaust manifold 54. A portion of the exhaust from the engine 52 is routed from the exhaust manifold 54 to an expansion turbine 56, which is coupled to a compressor 57 and which, together with the compressor 57, forms a turbocharger 55. Combustion air is drawn in and compressed by the compressor 57 and is routed to the intake manifold 53 of the engine 52 along a flow path 63. A charge air cooler 64 is arranged along the flow path 63 and allows for cooling of the compressed charge air by rejecting heat to a cooling flow 68. A remainder of the exhaust flow is directed from the exhaust manifold 54 to the intake manifold 53 along a recirculated exhaust gas flow path 65. An exhaust gas recirculation cooler 69 is arranged along the flow path 65 and allows for cooling of the exhaust gas by rejecting heat to a cooling flow 71. An exhaust gas recirculation valve 70 is additionally arranged along the flow path 65 to control the amount of exhaust that is being recirculated.
In such a system, the cooling stream 68 used to cool the compressed charge air is typically ambient air. To minimize the pollutant emissions, engine manufacturers typically target a charge air temperature into the intake manifold at several degrees above the ambient temperature, typically referred to as an intake manifold temperature differential or IMTD. Typical target values for IMTD are in the range of five to ten degrees Celsius. Achieving such low temperature differentials is most easily achieved by using the ambient air to directly cool the charge air, for example by arranging the charge air cooler 64 at the front of the vehicle where it can receive ram air from the forward motion of the vehicle.
Cooling the recirculated exhaust gas is more difficult, however, due to the elevated temperature of the exhaust exiting the exhaust manifold. These elevated temperatures can be damaging to surrounding components, and it is desirable to therefore locate the exhaust gas recirculation cooler 69 as close to the engine as possible, frequently directly abutting the engine. Use of ambient air as the cooling stream 71 is therefore frequently not feasible, and engine coolant is thus the predominant cooling stream used for the exhaust gas recirculation cooler. As such engine coolant is typically regulated to a temperature of around 100° C., the recirculated exhaust gas is typically delivered to the intake manifold at a substantially higher temperature than the cooled charge air, resulting in a mixed gas temperature within the intake manifold that is typically twenty or more degrees Celsius above the ambient temperature.
In addition, care must be taken to construct the exhaust gas recirculation cooler 69 from a material capable of withstanding the high temperatures that are associated with the exhaust of an internal combustion engine. In most cases, this requires that the heat exchanger 69 is constructed of a stainless steel alloy in order to ensure appropriate life of the heat exchanger in such a challenging operating environment. Stainless steel is, however, undesirable as a material of construction due to its high weight and cost. Thus, there is still room for improvement.